The present invention relates to a PLL (phase-locked loop) detection circuit.
A PLL detection circuit is used to detect the sub-signal, which is the difference signal between the right and left channel signals, in an AM-PM type AM stereo receiver.
For such a PLL detection circuit, it is necessary that unwanted AC signal components included in the output of the phase comparator be sufficiently suppressed by making the frequency response characteristic of a so-called "loop-filter" used in the circuit narrow so that only the DC signal component corresponding to the phase difference of the input signal, i.e., the phase-modulated signal is outputted. To accomplish this, the loop filter must employ a large time constant.
However, if the response characteristic is made narrow by increasing the time constant of the loop filter, then the lock-in time which elapses from the instant that the PLL detection circuit is placed in the unlocked state until it again achieves the locked state is increased, as a result of which the PLL detection circuit does not operate at the desired point. If automatic sweep is employed, sometimes locking cannot be obtained. Therefore, it is necessary to suitably change the frequency characteristic of the loop filter.
Accordingly, an object of the invention is to provide a PLL detection circuit in which the frequency response characteristic of the loop filter can be smoothly switched, and in which the amount of induced distortion due to the switching is small.